"The General number format
In cells that have the default General number format, Excel displays numbers as integers (789), decimal fractions (7.89), or scientific notation (7.89E+08) if the number is longer than the width of the cell. The General format displays up to 11 digits, including a decimal point and characters such as "E" and "+." To use numbers with more than 11 digits, you can apply a built-in scientific number format (exponential notation) or a custom number format.
Regardless of the number of digits displayed, Excel stores numbers with up to 15 digits of precision. If a number contains more than 15 significant digits, Excel converts the extra digits to zeros (0)."
Can you be more specific about which operations, number of decimal places and how willing you are to try unique approaches?
In the past in other languages, I have programmed the three basic functions (not division) for numbers of unlimited length as long as they were text. My code then did what any 2nd grader would do, digit by digit manipulation and store the result as text. Very convoluted to do, but it worked.
Another solution would be to act on each number in part. Store the numbers in chunks and act on simliar chunks. This works for addition and subtraction but not multiplication or division.
Can you post some sample numbers you would want to add and subtract? How many digits do you need? Are the numbers integers or do they have decimal points? Can the numbers be stored as text in the spreadsheet?
Okay, as an example of the limitation I mentioned above...column B shows the numbers that were entered in column A. Notice how XL slashes the numbers off after 15 digits. This will apply to integer, and decimal values.